onPause/onResume-lifecycle

Since some networks recommend to manage onPause and onResume events, so it is good idea to let developers catch them (see issue [#101](https://github.com/mopub/mopub-android-sdk/issues/101), for example).

Also, we got some reports about big CPU usage while application is in background - this behaviour isn't permanent, so we weren't able to dig deeper, though my co-worker reproduced it couple of times accidentally. So we introduced little fix to trigger WebViews' onPause() and onResume() methods: we haven't mentioned the problem since then. It's not a proof, I know, but anyway this code seems to be reasonable.